Nottinghamshire 214 for 6 (Mullaney 54*, Clarke 40) beat Yorkshire 213 (Lyth 63, Ball 3-32) by four wickets
Nottinghamshire made it five wins out of five in the Royal London One-Day Cup as they swept aside Yorkshire by four wickets at Trent Bridge.
Brook made 39 but was followed back to the pavilion by his partner two balls later, for 63, heralding the start of a dramatic collapse with Jake Ball the pick of the Notts bowlers, taking 3 for 32.
A crowd of 14,478 celebrated Nottinghamshire's Annual Family Fun Day, a record attendance for domestic cricket at Trent Bridge, according to the club. Many of them had barely taken their seat before Notts struck a major blow, with Matt Carter removing Tom Kohler-Cadmore for nought, after the Vikings had opted to bat first.
Lyth and Brook rebuilt with some purposeful running and crisp driving, although the former should have been caught in the deep by Joe Clarke on 35.
Returning for a second spell, Carter undid Brook with a clever piece of bowling. The off-spinner followed a loopy, well-flighted ball with a quicker one that burst through the defences and bowled the right-hander.
From the other end, Samit Patel had Lyth - who had reached his 50 from 44 balls, with five fours and two sixes - caught behind when cutting.
Wickets fell at regular intervals from then on. Gary Ballance was starting to look set until taken by a magnificent leaping catch on the boundary by Carter, off Ball.
Luke Fletcher added two more wickets, taking his tally for the competition to 15 already, as the Vikings innings folded with 7.4 overs remaining.
Only a flurry of early wickets was likely to derail the Outlaws' run chase but Chris Nash and Clarke offered little encouragement in adding 75 in the opening 10.5 overs.
A brilliant one-handed catch by Pillans ended the stand, sending back Nash for 35. Pillans sent down a fiery burst, which removed Ben Duckett and Jake Libby with consecutive deliveries, before also pinning Clarke lbw for 40.
From 121 for 4, the hosts were indebted to their skipper for guiding them through to the finishing line. Mullaney reached his fifty with two huge blows off Pillans, getting there from 55 balls, with four fours and three sixes.
The victory keeps Notts ahead of the chasing pack at the top of the North Group, with Yorkshire's hopes of advancing to the next phase now only a mathematical possibility.