The 38-year-old played his final game for the county in their 17-run victory over Warwickshire at Taunton.
"I'm quite emotional, when I went out to bat it felt like five pounds in my hand," Thomas said. "It's been eight wonderful years, you have to crack on."
Cape-Town born Thomas took 306 wickets in 91 first-class matches following his arrival at the club in 2008.
He enjoyed a golden summer in 2010, taking more than 100 wickets, and also scored 1,852 runs for the county in total.
The South African, who played one Twenty20 international for his country in 2007, is also the fourth highest T20 wicket-taker of all time with 263 wickets.
Speaking to BBC Somerset, Thomas picked out several highlights during his time at Taunton.
"There has been a couple of stunning games," he said. "We've chased 400 twice, there have been some great T20 games and some fantastic Friday night games to be a part of."
In 2014, Thomas also took a club record four wickets in four balls in a County Championship match against Sussex.
"Four in four was a personal highlight but the most important thing was the team won," he added. "I'm happy to take 0-100 as long as the team wins.
"All eight years have been a highlight. I've thoroughly enjoyed it, this has become home to me and the family. I have three kids in school and my wife works in the town, but it has to come to an end some time and the club has obviously seen that they have to move on but that is fine."
Thomas intends to stay in the game and may also extend his career elsewhere.
"I'd like to stay in cricket, I've not decided to call it a day with my playing career, I'm going to re-assess," he said.
"I've got a couple of coaching gigs happening and Millfield School asked me to fall in with them, starting next week so I'm looking forward to playing some part in helping the youngsters.
"The IPL [Indian Premier League] might come up again, other that I'm going to take stock and enjoy a bit of football with my youngster on a Saturday like normal fathers do and see how much that bores me. Hopefully not too much."
Somerset seamer Alfonso Thomas said he struggled to contain his emotion as his eight-year spell at the club finished.