The club also reached agreement on a four-year deal with cornerback Eric Wright, a second-round pick, leaving quarterback Brady Quinn as the team's only unsigned draft pick.
Thomas' agent, Peter Schaffer, who also represents Wright along with partner Lamont Smith, had been "grinding" through contract language with the Browns all day. Cleveland avoided a holdout by Thomas, the highest selected player to come to terms this summer.
The deal for Thomas can be shortened to five years. Financial terms weren't immediately available, but reports said the former Wisconsin All-American will get about $23 million in guaranteed money - the most ever given to a tackle.
"The deals for Joe and Eric are a culmination of three months of constant, good-faith, arms-length negotiations by myself, my partner and the Browns," Schaffer said. "There were countless hours of give and take, honest negotiations and hard work that led to two win-win contracts."
By getting the deals completed, Thomas and Wright will be on the practice field Friday afternoon when the Browns have their first workout with their veterans.
Unless something major develops in the next few hours, Quinn won't be there and any absence will hurt his chances of beating out Charlie Frye or Derek Anderson for the starting job.
The 6-6, 315-pound Thomas missed the team's rookie orientation program this week as Schaffer negotiated. Rated as the top lineman in April's draft, and one of the most polished to come out of college in the past few years, Thomas will compete with incumbent Kevin Shaffer for the starting job at left tackle.
Wright, who slipped into the second round due to off-field troubles he had as a freshman at Southern California, will compete for a starting job in the Browns' secondary. Financial terms were not available on the deal for Wright.
Now that the Browns have Thomas and Wright signed, they can turn their full attention to Quinn, whose contract situation is a tricky one. The Notre Dame QB slid through the first round before he was picked by the Browns, who traded a first-round pick in 2008 to Dallas to move into the No. 22 spot to get Quinn.
However, Quinn's agent, Tom Condon, who was in Cleveland for face-to-face talks with general manager Phil Savage last weekend, could be asking for his client to get paid like a Top 10 pick.