Button lapped the Spanish circuit in 1m 18.871s to finish over 0.2s ahead of new Renault signing Robert Kubica.
The reigning F1 Champ spent the day evaluating set-ups and assessing tyre durability in the MP4-25, covering a total of 108 laps.
Kubica, focused on long runs in the Renault R30 and said that it was a productive test.
“It was a good day and we tried a lot of different things, which has given us lots of useful information,” Kubica said.
“To begin with we did some evaluation with different fuel loads to check the balance of the car and then in the afternoon we did long runs with heavy fuel to simulate a race distance. We completed most of the program and I was pretty happy with the car.”
Third quickest was Kamui Kobayashi in the BMW Sauber C29, lapping the Spanish circuit 117 times in a trouble-free run.
Force India's Vitantonio Liuzzi ended the test fourth, covering 80 laps without any issues.
Friday's quickest runner Mark Webber ended the test down in eighth in his Red Bull Racing RB6 after losing time due to a blown Renault engine.
“We did a lot of stuff with Renault and also learned about the aerodynamics and the chassis,” he said.
“We need a nice clear run in Barcelona (the next scheduled test). A little more reliability there and we're ready to go racing.”
Nico Hulkenberg (Williams) and Timo Glock (Virgin), while Jarno Trulli had his first drive of the new Lotus T127.
“It was a very positive day,” said Trulli.
“I'm really happy about my first day testing with Lotus Racing and I was very lucky to have everything right today with no major issues. We were able to put a lot of mileage on the car and get a lot of information out of it. I had the chance to start developing a real feel for the car and it's good to be here.”
Testing resumes at Barcelona's Circuit de Catalunya next week, with another four-day session commencing on Thursday.