Johnson is one shot off the lead at the PGA Tour event in Silvis after an eagle-birdie finish on Thursday (Friday AEST) for a 7-under 64.
Lucas Glover and Zac Blair played in the morning and each posted a 63, a good start for both as the PGA Tour heads into the final two months. Both are well outside the top 100 in the FedEx Cup.
Johnson, a 50-year-old in his first year on the PGA Tour Champions and already with two victories, never considered anything but the John Deere Classic even with the US Senior Open in Ohio this week at Scioto.
He used putter from off the green for a 45-foot eagle on the par-5 17th, and capped off his round with a shot into 12 feet for a closing birdie.
"I just love being here, and I'm comfortable, obviously," said Johnson, who won the John Deere Classic in 2012.
"My scorecard ... I know that's an ingredient this week, but it's not everything. I can't stress it enough. I'm just appreciative of having a partnership with John Deere and feeling like a long, long-distance son to this area.
"A lot of affinity, a lot of affection."
Along with skipping the US Senior Open this week, Johnson did not enter the British Open at Royal Birkdale, where he is exempt for 10 more years. He also is skipping the Senior British Open at Gleneagles, where he played in the 2014 Ryder Cup.
Instead, Johnson is playing a senior major next week at Firestone. His reasoning for skipping the two British events is because he didn't think he could play for four weeks in a row.
The immediate goal is to play four rounds on the TPC Deere Run, where hot weather is expected to yield thunderstorms over the next few days.
Glover, who goes on the PGA Tour board next year, is at No.119 in the FedEx Cup. His exemption from winning back-to-back weeks in 2023 — including a FedEx Cup playoff event — ends after this year. Blair is at No.160 and has been playing where he can, along with Korn Ferry Tour events.
Stephan Jaeger and Lee Hodges joined Johnson at 64. Jaeger is at No.60 in the FedEx Cup, wanting to be sure he's among the top 70 in six weeks to qualify for the postseason. The BMW International Open in Munich is this week on his home course of Eichenried.
Jordan Spieth, playing the Deere for only the second time since winning in 2015, had a double bogey on the 18th hole in the middle of his round and closed with a bogey for a 69.
Karl Vilips is the only Australian in the field and is tied for 101st after the opening day.