Two goals in three first-half minutes from Michael Higdon and Nicky Law put Motherwell in command and Jamie Murphy scored the pick of their goals with a volley just after the interval.
Liam Craig pulled one back from the spot but substitute Henrik Ojamaa netted two late goals on the break as Motherwell moved five points clear in third place in the Clydesdale Bank Premier League.
Dean Shiels scored twice in the last 10 minutes to give
James Fowler put the hosts in front after 14 minutes, but his effort was cancelled out by Billy McKay's two goals in six minutes which gave the visitors a 2-1 interval lead. Inverness midfielder Jonny Hayes was sent off for a second yellow card just before the hour and Kilmarnock quickly capitalised, equalising through Michael Nelson.
Shiels netted twice in quick success in the closing stages to give the hosts some breaking space, although they had to endure a tense injury time after allowing Steve Williams to reduce the deficit to 4-3 right on 90 minutes.
Andy Kirk scored twice as the league’s bottom club
Kirk headed in a 68th-minute cross from Joe Cardle, who had put the Pars in front 12 minutes earlier, before capitalising on a defensive mix up for a third after 77 minutes.
The result saw Jim Jefferies' men end a 12-match winless run, but more importantly, cut the gap to Hibs, who play St Mirren on Sunday.
Willo Flood gave the hosts the lead in the 32nd minute, but within seconds Rudi Skacel got the visitors back on terms.
Again, United took control as Gary Mackay-Steven netted just after the hour mark, but substitute Arvydas Novkiovas levelled with seven minutes remaining.