First-half goals from Lionel Ainsworth and John Sutton left Well on the same tally as Aberdeen in the Scottish Premiership, while Caley Thistle were beaten again, just days after their shootout defeat in the Scottish League Cup final.
The visitors suffered an early scare as Stevie Hammell's low cross flew across the box, with Sutton stretching for a telling touch, but Inverness also threatened when Billy McKay's shot tested Gunnar Nielsen.
It was then Dean Brill's turn to show his agility when he turned away Ainsworth's volley, but the winger (26) was not denied for long as he raced onto Stevie Hammell's pass and fired inside the near post.
Hammell was the creator again as Motherwell added to their lead, chipping a tempting ball into the six-yard box for Sutton (34) to head home.
Caley's defensive woes continued after the break and Shaun Hutchinson glanced wide from Ainsworth's cross, but the Well centre-back then saved his side at the other end, hacking Greg Tansey's volley off the line.
Graeme Shinnie's drive was smothered by Nielsen as Inverness made a fight of it and they endured further frustration when substitute Ryan Christie dragged the ball wide.
Caley could not break Motherwell's resolve with some late balls into the box and only goal difference separates the Steelmen from second spot.