Playing for the first time since finishing tied for 40th at The Open at Royal Birkdale almost a full month ago, McIlroy was level par going into the final three holes before a disastrous streak left him towards the rear of the field.
Dropped shots at 16 and 17 were followed by a double bogey at TPC Southwind's final hole, which came after McIlroy found the water with his tee shot to the closing par four.
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The 74 left McIlroy nine shots adrift of the early leaders, with American duo Jordan Spieth and Michael Thorbjornsen setting the early pace with 65s.
England's Tommy Fleetwood, who was paired with Spieth, was a shot off that mark after opening up with a 66 that included a delightful chip in for eagle at the par-five third hole.
Fleetwood said: "I feel good. I didn't really play very good in the practice days. Game's not really been that great. I've been showing a lot, like some good and some really not so good.
"Hit it great off the tee today. Still my iron play was iffy, but I did a good job of managing it for the most part. Still hit some really good shots, and you can never underestimate how good - I think shooting under par on this golf course is very good."
World No 1 Scottie Scheffler, who played alongside McIlroy in the marquee group for the opening two rounds, put himself in a solid position by delivering a 68.
Scheffler said: "I felt like I hit a lot of quality shots with my irons.I did a good job of being patient and not trying to force it too much.
"I feel like I was right on the edge of having a good day, kind of all day is how I would describe it. So, I felt like patience-wise, I did a pretty good job."
Fleetwood had European company towards the top of the leaderboard, with Ludvig Aberg and Sepp Straka both joining Scheffler on two-under.
McIlroy could at least take consolation from not having the worst round of the day, which belonged to Scotland's Robert MacIntyre, who shot a 12-over par 82.
Memphis is hosting the first of three FedExCup Playoffs events, with only the top 50 in the overall season standings at the end of Sunday's final round progressing to the BMW Championship, and then the top 30 qualifying for the season-ending Tour Championship.
McIlroy's position of 12th out of the 70-player field means there is no concern about his progress to the BMW Championship, while the fact the events have no cut gives him plenty of time to climb the leaderboard over the weekend.
How do the FedExCup Playoffs work?
All season long, players have earned points based on how they have performed in PGA Tour events, with extra points available for the signature events and majors.
Only the top 70 in the standings have reached the FedEx St Jude Championship (August 13-16), with just the top 50 then progressing to the BMW Championship (August 20-23) and then only the top 30 making it to the finale, The Tour Championship (August 27-30).
Winners of the first two play-off events earn 750 points apiece - the same as The Players and the majors - before the points reset to zero ahead of the Tour Championship.
In the closing event, all players begin at even par, with the starting-strokes model based on FedExCup placings at that point axed in 2025.
The winner of the Tour Championship becomes FedExCup champion for 2026, earning $10m (£7.4m) and a five-year PGA Tour exemption.
Live coverage of the first event of the PGA Tour's FedExCup Playoffs, the FedEx St. Jude Championship, continues at 5pm on Friday. Watch live on Sky Sports Golf and Sky Sports Main Event or stream with no contract.
