The Leeds-born stand-off appeared 25 times for the French giants, and there were highs - wins over Toulouse and Bordeaux, champions of France and Europe, respectively - and some lows along the way.
"Some ridiculously tough times," is how he puts it in The Scotsman. His dad, Stuart Lancaster, was appointed head coach of Racing in 2023 and signed Dan last summer from Ealing Trailfinders. Racing struggled to get going and Lancaster senior was sacked in February with Racing 12th in the Top 14. Dan stuck with it until the end of the season but found out in May he wouldn't be retained.
"It's a really long story, but at the end of May I found out I wasn't staying at Racing for one reason, and that was that they wanted to get rid of the Lancaster name," he explains. "Then I got a call from Franco and it was very smooth sailing and easy. It was one conversation, the manager rang me and I said, 'I want to come to Glasgow', so it was easy."
It may not have ended in the manner he wished but Lancaster had proved himself in what is widely regarded as the world's strongest league.
"Obviously you want the team to be successful, but it was an invaluable experience," he says. "Playing so much rugby in a league which is very well respected, it's huge. Playing loads of games, playing in the stadiums, playing the kind of players, learning off people around me like Owen [Farrell] it's been massive for my career.
"Now it's time for me to transfer that onto the pitch and not just learn off someone, but help someone else learn, help a young lad like Matty Urwin [Glasgow's Scotland U20 international stand-off]. It was an amazing experience, I loved it.
Dad Stuart - who will be coaching at Connacht next season - is a former England head coach but he played age-grade rugby for Scotland, qualifying through his mother from Dumfries. Dan played for Scotland U18s and then England U20s and the pull of a potential Scotland cap was part of the attraction of joining Glasgow.
"Yes, for sure. I'd be lying if I said it wasn't," said Lancaster. "I'm Scots qualified through my grandma who's from Dumfries. It would be a massive honour to represent Scotland. For me, in the short term, it's just gaining respect of teammates, coaches and making the most of it if I get a chance."
Lancaster has had conversations with Gregor Townsend and the national coach said he was unlucky not to make the Scotland squad for the summer tour. Lancaster knows he has to prove himself with Glasgow before he can think of Scotland.
"I've not done anything yet. We've only played one pre-season game [a defeat at Bath at the weekend]. We've got another one on Friday [at home to Northampton], which is massive. There's so much rugby to be played before even thinking about [Scotland]. I'm so far away from thinking about it. I just need to get my head down here. I'm still writing calls down on my hand! I'm still playing catch-up.
"Glasgow reminds me of Leeds," he said. "Everyone's been welcoming, the lads have been great, I've been getting to know the coaches, so now it's just transferring it to the pitch and hopefully using the pre-season games to kick on into the season."