Or, if you read the books, like everyone did in the 70s, he's the funny/a-bit-naughty (not in that way) Uncle Ian to the more kid-friendly Uncle Terry (despite being the one whose last name isn't "penises").
Ian was a struggling writer/director in The Legitimate Theatah. When a friend who was going to try out for the part of Mike Yates couldn't make it, he went on a lark, and got the role. He still wanted to be a writer/director, so he turned it down and asked them to give him two guest spots instead.
His first guest spot was in Carnival of Monsters. His second one was supposed to be in the one that would have killed The Mattress if badly-maintained Turkish roads hadn't done that first. But the show desperately needed a new Ian Chatterton action man, because they were going to hire an old and feeble actor for the Fourth Doctor, and they talked him into taking the role, Harry Sullivan. And then they hired Tom Baker instead, and had no use for Harry, so he was quickly dumped.
He did a few more minor guest acting roles after that for BBC shows and post-Hammer horror film companies, but even he didn't care about those, so I'm not going to list them.
Ian tried to talk himself into becoming a writer for the show, in hopes of taking over as script editor, but failed at that. But then he heard Tom Baker drunkenly ranting about how he knew important movie people and could get a movie made, so he co-wrote Doctor Who Meets Scratchman with Tom. This would have turned him into a world-famous screenwriter, except that it turned out Tom didn't really know important movie people, and was planning to fund the film by Kickstarter until he learned that Kickstarter didn't exist yet.
But then Ian discovered he could get a job writing Target novelisations. And everyone loved him at that. Mainly because he used slightly naughty words, so you could get away with running around the house yelling "bastard! bastard!" and if mum got mad you could say "But Doctor Who said it!" and she'd write a letter to the BBC instead of grounding you.
He's also basically the grandfather of the Virgin New Adventures twice over (hey, this is England, you have to expect a bit of crossed branches on the family tree). According to Marc Platt, Ian was the first writer who intentionally expanded on the TV stories to improve them (instead of changing them because he only vaguely remembered the script and couldn't be arsed to check). And he also badgered Target into creating a line of original Doctor Who stories and then wrote one of the two, Harry Sullivan's War, before that line was canceled.
Meanwhile, Ian discovered that there was quick money to be made writing novelisations for Hollywood movies like Splash and Down and Out in Beverly Hills, so he did that (under the imaginative pen name Ian Don) to keep food on the table, while doing more Targets for fun, and writing the greatest Doctor Who novel ever (a quasi-sequel to Scratchman) for high art.
Unfortunately, he died of an unexpected heart attack in his 40s years before Douglas Adams made it trendy, so his great novel was never finished.