McCague's six-wicket haul put Kent firmly in the driving seat on a day in which 19 wickets fell. Umpires David Constant and John Hampshire will report the pitch - something they are required to do if 15 or more wickets fall in a day.
At least four of the Kent wickets, after Steve Marsh won the toss and elected to bat, were lost to poor shots.
David Fulton and Matthew Fleming both hoisted high hooks to the deep long-leg area to be caught comfortably while Alan Wells and Marsh were both caught off Phil Tufnell, top edging sweeps.
Ed Smith, who opened with Fulton, was beaten for pace as Angus Fraser embarked on a spell of three wickets in 19 deliveries for just 10 runs. Tufnell took 3 for five in 25 balls, to help finish Kent off.
Kent's 208 total looked vulnerable but McCague tilted the balance of the match.
Alan Igglesden, playing only his second Championship match of the season, took his first wicket for nearly two years when he had Jacques Kallis caught at first slip and, four overs later, he bowled Paul Weekes.
But it was McCague who did most damage. He had Mark Ramprakash leg before and bowled Jason Pooley in his first spell of 12 overs.
Ben Phillips dismissed Owais Shah to break the Middlesex fifth-wicket partnership before McCague returned and unleashed his four-wicket burst.
Durham were dismissed for 86 at Cheltenham - their lowest first innings score since they came into the Championship. They won the toss but a total collapse saw eight wickets go down for 25 runs before lunch.
Third-placed Gloucestershire responded by reaching 253 for 5 off 66 overs and their overall success was achieved in the absence of their leading wicket-taker, Mike Smith, who is on England duty.
The Tasmanian all-rounder Shaun Young filled the gap as he sent back openers Jon Lewis and Mike Roseberry in his first six overs but it was the Gloucestershire captain, Mark Alleyne, who demoralised Durham.
A 12-ball spell brought him the wickets of John Morris, David Boon and Nick Speak for three runs and he finished with 5 for 14 in 8.2 overs. Morris was top scorer with six fours in his 32 but Boon, out for a duck, has scored just 18 in his last six innings.Reuse content