The ever reliable Test left-hander, who's become a firm favourite with the Old Trafford-based county since joining last year, was dropped twice on a tricky morning at Chester-le-Street after Lancashire had been asked to bat by Durham.
But taking advantage of those reprieves even as wickets were falling around him, there still felt an inevitability about the 33-year-old 'Harry' carving his way expertly towards the 35th first-class hundred of his career and his fifth championship ton for Lancashire.
His five-and-a-half hour vigil was the bedrock of Lancashire's innings as they battled their way to finish the day on 7-356, with Harris still at the crease on 135no, having struck 19 fours and having shared a stubborn unbroken eighth-wicket stand of 64 with paceman Tom Bailey.
It followed the knocks of 125 and 66 that Harris had compiled in the opening game of the season against Derbyshire at Old Trafford.
It all left Lancashire's assistant coach Will Porterfield marvelling: "He's been incredible, carrying on from where he left off last season. We got a few stats sent through. It's the most runs (1364) anyone has scored in their first 24 innings in a Lancashire shirt. Long may it continue!"
Another former Test opener Cameron Bancroft has been looking for a change of fortune for the second division side he captains, with Gloucestershire having lost their first three matches of the season.
Bancroft had been going along nicely if sedately for 40 until he got caught at square leg, but Miles Hammond's unbeaten 140 then guided the visitors to 6-287.
At Trent Bridge, England fast bowler Josh Tongue took five Warwickshire wickets for defending division one champions Nottinghamshire, who were without their injured Australian quick Fergus O'Neill.
Tongue's scalps included Aussie Test allrounder Beau Webster, who was bowled for 25 in Warwickshire's 8-375 that relied heavily on Ed Barnard's captain's knock of 134no.
Joe Root made his first championship appearance of the season but was made to wait to get bat in hand as Yorkshire opted to bowl first and struggled to subdue Sussex, who compiled 6-373 in their division one match at Headingley.
John Simpson ended the day unbeaten on 101 out of 6-373, after Aussie Daniel Hughes had fallen early in the day for 10.
