Forbes lashed the ball home from 16 yards - his seventh goal of the season -as Well continued their unbeaten domestic record under Jim Gannon despite twice losing the lead to former manager Terry Butcher's Irn-Bru First Division side.
Well took control after Robert McHugh's first-half header but Inverness substitute Andrew Barrowman took advantage of some slack marking to head a 70th-minute equaliser, just before David Proctor was sent off.
An own goal from Nauris Bulvitis put Well ahead four minutes into extra-time and Forbes prevented penalties after Grant Munro had headed another equaliser.
The warning signs came early for Well as Inverness created the first chances in a scrappy opening quarter.
Dan Stratford shot wide after Motherwell had lost the ball from their own free-kick and Richie Foran's first-time volley flew well wide after a Jonathan Hayes cross.
Inverness kept Motherwell at arm's length until the half-hour mark when Proctor was booked for a cynical foul on Jim O'Brien.
Forbes curled in the free-kick and Lukas Jutkiewicz saw his downward header blocked by Ryan Esson. O'Brien then sent Jutkiewicz clear through the middle but the on-loan Everton striker found himself too wide after taking the ball round Esson and saw his shot blocked by the goalkeeper.
Well took a 33rd-minute lead from the same passage of play when McHugh - in for the injured John Sutton - guided Yassin Moutaouakil's powerful cross into the far corner.
Well full-backs Moutaouakil and Stevie Hammell were playing in advanced positions and Gannon's side were almost caught out when Tom Hateley lost the ball midway inside the Inverness half.
The visitors got the ball forward quickly and Russell Duncan had a clear sight of goal but the midfielder's shot trundled wide.
The lively former Well striker Foran curled over with the aid of a deflection just after the break but Motherwell soon took a grip.
Jutkiewicz turned and shot straight at Esson and McHugh volleyed wide off a defender, both after crosses by Moutaouakil, who soon cut inside and had a left-footed strike saved by Esson.
Saunders glanced wide from O'Brien's cross but the visitors grabbed a shock equaliser after Forbes had been harshly penalised for a challenge on Duncan.
The home defence slacked off as Hayes curled in the free-kick and the unchallenged Barrowman headed into the corner from eight yards.
Four minutes after levelling, Inverness found themselves a man down when Proctor received his second yellow for chopping down O'Brien for the second time in quick succession.
But Esson forced extra-time with two excellent stops from chances created by Forbes.
The Inverness goalkeeper tipped Saunders' header over and somehow kept out McHugh's 82nd-minute header as the ball bounced along the line and spun out for a corner.
Motherwell continued to push forward as extra-time began. Moutaouakil had a shot parried over after being set up by Giles Coke and they went in front when Bulvitis sliced Hammell's low cross into his own net.
Well kept possession without threatening and they were again hauled back in the 110th minute when Grant Munro looped a header over John Ruddy following Hayes' corner.
Esson kept out Forbes' strike at his near post but he was soon beaten at the exact same place when the midfielder cut inside and lashed the ball into the roof of the net from 16 yards.
Esson still had time to save well from Jamie Murphy, Moutaouakil and Forbes as Well finished with a flurry.
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