Anime fandom in the US can easily be divided into the time before Toonami and the time after Toonami. Anime existed on US TV before, but almost always in heavily edited forms in syndication time-slots. If the people dubbing these early TV anime were occasionally fans (Carl Macek had to make some bizarre changes to sell Macross as Robotech, but he was genuinely a fan), the people who were marketing them certainly weren't. Just look at how Saban marketed their syndication broadcast of Dragon Ball Z, you can feel the contempt:
At Cartoon Network, though, there were a few anime fans. When the network started in 1992, they didn't broadcast anime, but in 1995 they experimented with a late-night anime movie event showing Vampire Hunter D, Robot Carnival, and Twilight of the Cockroaches. They also aired a few old, easy-to-acquire dubs like Speed Racer for their Powerzone afternoon action block. But it would be when they rebooted their afternoon block that things would get interesting.
1997 to 1999: The Moltar Era and the Anime Boom
Toonami first Intro Toonami Block Adult Swim Cartoon Network
Toonami was created by Sean Akins and Jason DeMarco as an afternoon action cartoon block. Hosted by a CGI version of Moltar from Space Ghost, it packaged old cartoons with techno music, video game reviews, skateboarding videos, and inspirational speeches. The goal was to make something that spoke directly to kids rather than down to them while indulging all the things the block's adult creators personally enjoyed. The first line-up only contained one anime series, Voltron, but in 1998, Toonami made anime fandom explode in a big way.
It's debatable if the big anime boom in America would have happened if Toonami hadn't gotten the rights to Dragonball Z and Sailor Moon, transforming two series struggling in syndication into mainstream phenomenons. Certainly Pokemon would have still been a break-out hit the same year, and Touchstone's already-planned 1999 release of Princess Mononoke (which Toonami promoted) would have still helped anime's worldwide critical if not commercial popularity. But Toonami bridged the gap. Rather than just toy commercials for kids or acclaimed but little-seen movies in the arthouse, Toonami was able to introduce kids to anime that were easily accessible but not just merchandising machines. Older viewers might have disliked the still-rough editing and dubbing, but coming at the same time as the internet and the DVD boom, it was the perfect gateway to an expanding anime fandom.
1999 to 2000: The TOM 1 Era, Better Dubs and Less Censorship
In the middle of 1999, Toonami switched from using Moltar as the host to its own original creation, a robot named TOM voiced by Sonny Strait. 2000 saw Toonami's most daring acquisition yet: the US TV premiere of Gundam Wing. A war story skewing towards the older end of their audience, the afternoon broadcast had the expected edits, but the late night showing on "The Midnight Run" marked the first time an anime TV series aired completely uncut with a faithful dub on US TV! The trailer Toonami made, narrated by Peter Cullen, was so good Bandai ended up using it as their official trailer for the DVDs.
Another noteworthy acquisition in 2000 was Tenchi Muyo. This one didn't air uncut due to nudity; it couldn't even on Adult Swim today! "Digital bikinis" aside, it's crazy a harem anime actually aired on a daytime children's TV block and was semi-comprehensible. By 2000, almost all anime received uncut DVD releases, so anyone who wanted to see the original version of the show had the opportunity.
2000 to 2003: TOM 2 and Adult Swim
In late 2000, Toonami sent the first TOM off with a bang and introduced the new host TOM 2 (voiced by Steve Blum, who'd voice all the later incarnations of TOM) with their first "Total Immersion Event", The Intruder.
Toonami had a few notable hits during the TOM 2 era. The Big O did so much better on Toonami than it did in Japan that Cartoon Network helped greenlight a second season! But In late 2001, Toonami itself started to play second-fiddle to a block that couldn't exist in the form it does if not for Toonami's influence: Adult Swim. While Akins and DeMarco could edit many shows to Toonami standards, even the likes of Outlaw Star, Cowboy Bebop just couldn't fit Toonami at the time, so they premiered it on Cartoon Network's new adult-targeted, late-night block instead. Due to Bebop's success, several anime that had been scheduled for Toonami were moved to Adult Swim (most of the early ones, unfortunately, still with Toonami editing).
This was the era of the block where the much-loved "Dreams" AMV premiered.
2003 to 2008: TOM 3, TOM 4, and the First Death of Toonami
Toonami updated the TOM CGI in 2003. A year later, the block moved from weekday afternoons to Saturday night. Shows like Rurouni Kenshin were attracting primarily teenage audiences, older than Cartoon Network wanted to attract but not quite in the Adult Swim crowd either. To deal with this, the block moved to Saturday nights, where younger-skewing shows early in the evening would transition to more mature fare later at night before leading into Adult Swim, while weekdays got the relatively short-lived Miguzi block as a replacement.
Later timeslots allowed for greater leniency in content. Reruns of the first season of Dragonball Z were permitted to air uncut with a TV-PG rating (instead of the typical TV-Y7), and Naruto, the biggest hit of Toonami's later years, was able to air with minimal editing, blood and stabbing intact. Toonami also aired several Ghibli movies uncut in a "Month of Miyazaki" event in 2006. And during the 2005 to 2006 season, Toonami produced its own original anime IGPX, a mecha racing series with a big animation and voice acting budget which unfortunately wasn't a hit.
All good things must come to an end, though, and Toonami experienced a prolonged decline from 2007 to its supposed final broadcast in 2008. The TOM 4 redesign, which gave the robot host a mildly unnerving face, wasn't a success. The block was cut in half from 4 hours to 2. New shows were rare. The logic behind Toonami's ultimate cancellation isn't totally clear. Even in the end, the block was still #1 in the ratings among boys 9 to 14 in its timeslot. But Cartoon Network was going through a difficult period, trying to distance itself from anime while disastrously experimenting with live-action programming, and at the very least, Steve Blum and the Toonami crew let the block go out with a "bang"!
2012 to Present Day: Toonami's Back, Bitches!
April 1st, 2012 is a day that will go down in infamy. Viewers watching to see what sort of crazy prank Adult Swim was going to pull this time were treated to the first few seconds of Tommy Wiseau's The Room, which had aired on April Fools Day in previous years... only to pull back and see TOM and his hologram co-host Sara on their TV again for the first time in years! The rest of the night was a nostalgia bomb, a mix of old Toonami shows and Adult Swim anime series, with a new review of Mass Effect 3 and a remastered version of the classic "Dreams" video.
The reaction to this prank was so intense, that in less than 2 months, Adult Swim decided to bring back Toonami for good. The new block would be different from the old Cartoon Network block; while it would include some shows from the old Toonami, it would be aimed at adult audiences (the new theme song for the premiere broadcast, Richie Branson's rap "#ToonamisBackBitches", and the first new shows, the ultra-violent Deadman Wonderland and the dark Casshern Sins, set the tone). Essentially it was a continuation of Adult Swim's long-running anime programming, but it was the first time Adult Swim had put effort into promoting their anime in quite some time, so it definitely counted as an improvement.
But how does Toonami stay relevant it once did now that most anime is available legally streaming? Well, in the case of Space Dandy in 2014, they managed to get premieres before Japan did. But even when they're showing older series, Toonami serves a purpose as a curator of anime for casual audiences intrigued but intimidated by vast streaming libraries. Toonami can get viewers for overlooked gems like Michiko & Hatchin and turn an already popular show like Attack on Titan into anime's biggest mainstream hit in America since Naruto. Even for fans who've seen it all, with its game reviews, music videos, and special events (they recently did a sequel to The Intruder), the atmosphere makes it pleasant viewing late on a Saturday night.
2016 is looking like another good year for Toonami. They've premiered the dub of Dimension W and have announced their first original cartoon of the Adult Swim era, a new season of the cult hit Samurai Jack. The current heads of the block, Jason DeMarco and Gill Austin, have teased they have more big announcements in the near future (a simuldub of Attack on Titan Season 2 seems like almost a sure thing, right?). You can keep up with their weekly webcast Toonami Preflight for all the latest news on where Toonami goes in the future.