Chris Couch finished one shot further back at 6-under-par 66 along with D.J. Brigman, Jason Dufner and Bo Van Pelt.
With plenty at stake at the season-ending event, Johnson has less to worry about than his competitors. The 27-year-old, a two-time winner this season, leads the money list and is well on his way to the PGA Tour.
'Granted, I'm in a position where the stress is off, especially relative to most of the other guys,' said Johnson. 'The one thing I've done all year is not give up and not quit. I guess you could get complacent in the position I'm in and that's the last thing I want to do.'
Johnson parred the first four holes on the Senator Course at Capitol Hill before running off back-to-back birdies starting at the par-5 fifth.
He stumbled with a bogey at the par-3 seventh but recovered in stellar fashion with three consecutive birdies from the eighth.
Johnson then reached 5-under with a birdie at the par-4 15th to close in on the leaders.
After missing the green at the par-5 17th, Johnson chipped in for an eagle to grab the lead at minus-7.
'I had a good lie and just wanted to pitch it onto the green,' said Johnson. 'I figured if I could get it on, it would feed right down to the hole.'
Brigman started with an eagle at the par-4 third and added a birdie at the fifth. He faltered with a bogey at the sixth but ran off three straight birdies from the eighth to move to 5-under.
The 27-year-old bogeyed the 12th but dropped his tee shot inside 16 feet for a birdie at the par-3 13th.
Brigman then hit his approach to five feet for a birdie at the 15th to finish one shot off the lead.
'There are two ways to play this course. You can lay back and hit long irons into the holes or you can be aggressive off the tee and hit it over the bunkers,' said Brigman, who earned his first career victory at the Permian Basin Charity Golf Classic earlier this month. 'I've got nothing to lose this week. I just want to have a shot on Sunday.'
Tjaart van der Walt, Roland Thatcher and Ken Duke finished two shots off the lead at 5-under-par 67. Craig Lile, Jess Daley, Daniel Chopra and Roger Tambellini were one shot further back at minus-4.
Michael Allen, Scott Gump, Doug LaBelle II, Charles Warren and Andre Stolz tied for 13th at 3-under-par 69.
Joe Ogilvie, who is second on the Nationwide Tour money list, finished in a group at even-par 72.
The top 20 on the money list after the Nationwide Tour money list will earn their PGA Tour cards. Those who finish from 21-55 will secure their playing position on the Nationwide Tour in 2004.