Celtic full-back Andreas Hinkel tried his luck from distance but failed to test goalkeeper Luke Daniels, who was making his debut for the home side.
Daniels, on loan from West Brom, was then under pressure as Gary Caldwell rose to meet a Shunsuke Nakamura free-kick but the visitors could not capitalise on the keeper's fumble.
On 36 minutes, Motherwell's Darren Smith scampered down the right flank and crossed for Marc Fitzpatrick only for the midfielder to direct his strike straight at goalkeeper Artur Boruc.
Motherwell continued to look the livelier side after the break and grabbed a fine opening goal on the hour when McLean curled in a cross for Porter to head home.
However, Celtic replied immediately with former 'Well striker McDonald nodding in from Nakamura's lofted delivery to the far post.
And Gordon Strachan's team went ahead on 78 minutes when Samaras was left unattended at a Barry Robson corner and the substitute had the simple task of heading in from close range.
The home side, aiming to hold on to third spot, will feel aggrieved since it was a dubious decision to award the decisive corner but poor marking was their undoing for both goals.
Motherwell forced a series of corners in the closing stages but the visitors held firm to claim a vital victory.
Motherwell: Daniels, Brian McLean, Craigan, Reynolds, Hammell, Darren Smith (McCormack 74), Malcolm, Fitzpatrick, Lasley (Murphy 86), Porter, Clarkson (Hughes 81). Subs Not Used: Graeme Smith, Meechan, Lappin, McGarry.
Booked: Malcolm.
Goal: Porter 60.
Celtic: Boruc, Hinkel, Caldwell, Balde, Naylor (Wilson 75), Nakamura (Scott Brown 65), Hartley, Robson, McGeady, McDonald, Vennegoor of Hesselink (Samaras 65). Subs Not Used: Mark Brown, O'Dea, Donati, Hutchinson.
Booked: Robson.
Goals: McDonald 62, Samaras 79.
Att: 9,158
Ref: S Conroy
BBC Sport Player Rater man of the match: Motherwell's Darren Smith 7.15 (on 90 minutes).
Please note that you can still give the players marks out of 10 on BBC Sport's Player Rater after the match has finished.
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