Worcestershire 223 for 2 (Mitchell 80, Libby 77) trail Gloucestershire 267 (Dent 92, van Buuren 60, Morris 4-52) by 44 runs
Playing together for the first time, the pair staged a superb stand of 152, veteran campaigner Mitchell top-scoring with 80 and Libby making an impressive 77 on debut as Worcestershire reached the close on 223 for 2, just 44 behind with eight first-innings wickets in hand.
On a day when they might easily have wilted, Gloucestershire's bowlers stuck to their task admirably, Tom Price and David Payne making crucial interventions either side of the tea interval to remove Mitchell and Libby respectively.
Tom Fell and Jack Haynes reached the close unbeaten on 32 and 20 respectively and Worcestershire will have 38 overs more in which to build a lead on day three.
At one point in mid-afternoon, it looked as though Worcestershire's new opening pair would both go on to register hundreds on an increasingly docile surface. An authoritative presence, Mitchell was first to 50, reaching that landmark via 101 balls with nine fours.
Making up for lost time after waiting nine months to make his debut following his switch from Nottinghamshire, Libby adopted a supporting role until plundering three boundaries in one Ryan Higgins over to pass 50 in a flurry.
Their partnership realised three figures in 37 overs and, when Gloucestershire skipper Chris Dent turned to spin, Graeme van Buuren was hit out of the attack.
Instead, it was Academy product Price who made the much-needed breakthrough, Mitchell dragging a bottom edge onto his stumps in the act of pulling to provide the youngster with his maiden first class wicket on debut. Having faced 161 balls and accrued 13 fours to move to within sight of what would have been a fifth hundred made in matches against Gloucestershire, Worcestershire's most experienced batsman was understandably frustrated at the manner of his dismissal.
Cornishman Libby scored a hundred on debut for Nottinghamshire against Sussex at Trent Bridge in 2014 and was intent upon becoming only the twelfth player to do so for Worcestershire when he became becalmed in the face of tight bowling after tea. Losing vital momentum, he was caught in the slips when pushing at a slanting delivery from David Payne as the visitors slipped to 180 for 2.
He may have missed out on a coveted hundred, but Libby showed why Worcestershire were so keen to recruit him last winter in a well-organised innings which spanned 181 balls and included 10 fours.
Gloucestershire's quintet of seamers mustered a disciplined line and length in a final session which yielded a mere 62 runs in 29 overs.
Required to rebuild the innings and set a base for the big hitters to come, Fell and Haynes survived without ever really kicking on.
Resuming on 246 for 8, Gloucestershire's tail-end resistance lasted just 30 minutes in the morning. Josh Shaw departed to the fifth ball of the day, Josh Tongue locating his outside edge and wicketkeeper Ben Cox taking an excellent tumbling catch low down in front of first slip.
Determined to make a good impression against Worcestershire, the county he has agreed to join next year, Gareth Roderick mustered lower-order resistance, plundering three boundaries off one Morris over to secure a second batting bonus point and hasten the advent of the new ball.
Morris removed Matt Taylor shortly afterwards, leaving the home side to reflect upon a dramatic decline which saw their last eight wickets go down for the addition of 62 runs inside 27 overs.
Of Gloucestershire's middle and lower order batsmen, only Roderick emerged with distinction, finishing unbeaten on 25, his resistance spanning 41 balls.