Mercilus was the 26th overall pick in this year's draft out of Illinois.
"It's great to get it off my mind and just concentrate straight on football and everything," he said. "Just more concentrating instead of having a monkey on my back, no doubt and just getting in front of the fans."
The 6-foot-4, 254-pound player was a defensive end in a 4-3 scheme at Illinois but has moved to outside linebacker in the 3-4 alignment the Texans use. He's happy with the progress he made in the practices leading up to this week's mini-camp, which wrapped up Thursday.
"I've grown up a lot, pretty fast, too," Mercilus said. "They shot it straight at us really fast, a hundred miles per hour and it's a lot of information to take in. But it was our time for these three days of the mini-camp to actually grow up and graduate and basically go out here and execute what they call."
He started just one season in college, but was the nation's leader in sacks with 16 and forced fumbles with eight. He earned Big Ten first-team honors and the Ted Hendricks Award as the top defensive end in the nation.
The Texans did not release financial terms of the deal.
Also on Thursday, Houston signed free agents Keith Browner and Moran Norris, who spent the past three days at mini-camp trying out for the team. Browner, a defensive end and linebacker, played in 38 games in his past three seasons at California.
Norris, a 12-year veteran fullback, played for the Texans from 2002-05 before spending 2006 and 2007 with the 49ers. He spent the 2008 season with the Lions before returning to San Francisco for the last three years.