Steve Taylor opened the bowling for Mildenhall in place of the absent Jon Steel, and found early success when Andy Squire hung onto a sharp slip catch to dismiss Charlie Douglas-Hughes for five.
Josh Wells came into the crease alongside Ollie Grayson for the hosts, who just after reaching his half century saw his partner run out for 42.
Mildenhall were unable to contain Wells as he plundered runs all around the ground, but did dismiss Mark Johnson and Waj Bukhari to leave Halstead on 147-4.
Dylan Verreynne (67) was removed after lofting a ball to point off Taylor and Squire held on, but it did not stop them passing the 300 mark and Wells beyond 150 runs.
Joe Reed caught Chris Rayner for a duck and Ben Shepperson caught Wells (201), before Halstead gifted Mildenhall a couple of points as the wickets of Harry Veal and Darren Hutchinson fell to finish on an imposing 345-9.
Taylor struck two big sixes at the start of the run chase as the visitors raced to 50, before the Mildenhall opener was caught for 38.
Shepperson (70) joined Reed at the crease and the duo added 114 in 18 overs, but Wells delivered another blow with a one-handed catch to dismiss the Mildenhall captain.
Reed departed for an excellent 145 as they remained on target for victory, but a flurry of wickets in the final overs saw Mildenhall finish just short on 335-7.
n A fine all round performance from Peter Worthington (73 not out, 4-15) helped Mildenhall II beat Copford, while Jordan Beaumont (61, 5-40) was the hero in Mildenhall III’s three wicket victory over St Margarets.
n Lou Handy (59) and Martin Kingdon (5-18) led Mildenhall IV to a 130 run win over East Bergholt, while Mildenhall Sunday remain top courtesy of Jack Loveday’s unbeaten knock of 90.