Two second half goals by Devante Cole and Stephen O’Donnell earned the Steelmen a win which moves them above St Mirren and Dundee United into seventh place.
An excellent chance was spurned by Motherwell on eight minutes when a fine build-up culminated in Nathan McGinley’s precise cross from the left being headed over by Jake Hastie.
Hastie’s 16th minute corner from the right fell to Sam Foley whose first time shot from outside the box wasn’t too far wide.
As the rain battered down torrentially, the visitors’ first real chance then arrived when Kirk Broadfoot nodded wide after a Greg Kiltie corner.
Lawless’s short corner at the other end then saw Hastie’s delivery met by Ricki Lamie whose shot was blocked.
A Killie free-kick from the right was then cleared to Alan Power whose shot was straight at Liam Kelly.
The final first half action saw Declan Gallagher’s long ball forward chased by Hastie who appealed for a penalty after going down in the box under a challenge from away keeper Danny Rogers, but referee Bobby Madden wasn’t interested.
Cole shot home superbly with his right foot on 47 minutes after good set-up work by Foley and Long.
Another fine ’Well move saw Long switch the play right to Hastie who sprinted forward before shooting into the side netting.
Kyle Lafferty – back from inury for the visitors – was then put through on goal but Lamie’s presence led to him trying an ambitious chip which went over the bar.
But Lamie then made a mess of a backpass to Kelly and the ball broke to Lafferty whose long range effort was deflected just wide with Kelly beaten.
Kelly then made a great diving save from Lafferty’s volley after an initial cross from the right had been cleared.
Killie were well in the game now and substitute George Oakley headed disappointingly high and wide after Brandon Haunstrup’s delivery from the left flank.
The visitors’ Ross Millen then found substitute Chris Burke whose long range dipper hit the top of the bar.
It was 2-0 on 85 minutes when brilliant set-up play by Cole saw him cross for O’Donnell to drill the ball home from 12 yards.
Watt set up Barry Maguire late on and his powerful left footer wasn’t too far wide.
Motherwell: Kelly, Lamie, McGinley, Campbell, Long (Maguire 79), Hastie (Watt 64), Lawless (Mugabi 72), Gallagher, O’Donnell, Foley, Cole.
Kilmarnock: Rogers, Haunstrup, Broadfoot, Power (Oakley 59), McKenzie (Burke 59), Dicker, Kiltie (Tshibola 59), Ibsen Rossi, Pinnock, Millen, Lafferty.
Referee: Bobby Madden