On-loan Notts County midfielder Joss Labadie got the relegation-threatened Gulls off to a flying start with his second-minute goal, heading home a cross from forward-thinking left-back Thomas Cruise.
United's joy was doubled six minutes later, on their second attack, as winger Jordan Chapell's thumping goalbound drive was back-heeled past Spireites keeper Tommy Lee by target man Howe.
Man-of-the-match Howe then sent a 25-yard shot screaming past Lee, skimming inches past the post, before heading just wide in the next attack.
Labadie and on-loan Southend striker Elliot Benyon both went close before home keeper Michael Poke went full stretch to palm over Conor Townsend's thumping free-kick strike.
Chesterfield set United nerves jangling when Liam Cooper headed home sub Mark Randall's cross in the 77th minute.
But United held firm with central defenders Brian Saah and Angus MacDonald resolute in front of busy Poke, giving Alan Knill his first win in charge.