Support from Ayush Patel (46), Andy Hibberd (34 no) and Thomas Shorthouse (32) saw the visitors post 292 for 4.
Chasing a revised target of 266, Grantham were limited to 162 for 9 off their 42 overs.
Zain Abbas top scored with 64 while Sleaford's Joe Peck and Henry Cooper took three wickets apiece.
Sleaford now lead Grantham by eight points but trail leaders Bourne - who beat Woodhall Spa by five wickets - by seven.
Oliver Caswell top scored with 47 as the Spa men were dismissed for 101 at Abbey Lawn.
Jack Berry claimed five wickets for the loss of 23 runs before his side reached 104 for 5 in the 41st over.
Carl Wilson (34 no) and Jonathan Cheer (22) top scored for Bourne, Woodhall's Alex King taking three wickets.
Forth-placed Lindum were beaten by 18 runs as they hosted Market Deeping.
S. Malton (29) and J. Smith (34) led the way as Deeping posted 173 before being dismissed while 44 from G. Robbins helped Lindum to 155 all out.
M. Mills, N. Niazi and J. Smith each took three Lindum wickets while Lindum's A. Harris also claimed three victims.
Remembrance Nyathi (57) and Samueal Jones (47) led the way as Louth beat visitors Scunthorpe Town by nine runs.
Ian Snowden hit an unbeaten 86 for Scunthorpe, who fell just short.
Louth's Stephen Kirkham took three wickets.
Kit Spence scored 121 runs for Bracebridge Heath in their victory over bottom side Skegness.
Sam Mason added good support with 83 while Jamie Epton's 59 top scored for the seasiders.
Bracebridge's Alex Moor took three wickets in the 88-run success.
Spalding secured a 14-run home win against Nettleham.
Mitch Freeman (44) top scored for Spalding with the visitors' Rhett Bridgens and Harry Thornton taking three wickets each.
Spalding's Brandon Andrews also took three wickets while Nettleham's Harry Pickett (33) was the pick with the bat.
Results: Bracebridge Heath 279-6, Skegness 191 all out; Louth 183-6, Scunthorpe Town 174-9; Spalding 127 all out, Nettleham 113 all out; Bourne 104-5, Woodhall Spa 101 all out; Grantham 162-9, Sleaford 292-4; Lindum 155 all out, Market Deeping 173 all out.