Ragged Saints, who are under the interim charge of Eamon O’Carroll for the rest of the season after Paul Rowley’s exit last week, have lost eight of their past 12 matches.
The 10-time Super League winners remain in seventh place and are in real jeopardy of missing out on the play-offs for the first time.
Saints had looked dead and buried when Field went in for his second try after a lovely offload from Jack Martin but the visitors were only a missed conversion from Tristan Sailor from taking the game to golden-point extra time.
Even then they had time for one last play but a forward pass brought it to an end and meant they lost to York for the first time since 6 April 1946.
The error-strewn tone of the game was set after just three minutes when York failed to deal with a high kick from Jackson Hastings and Davies snaffled the loose ball to put the away team in front.
Ex-Saints man Bennison and then Field were beneficiaries of penalties to put the home side ahead only for another handling error to let Wright in and then Welsby avoided a last ditch tackle as O’Carroll’s men went back in front.
Balmforth went in from dummy half to level the scores at 16 apiece at the break but Mark Applegarth’s side’s hopes were dealt a blow when Hingano was sin-binned for a high tackle.
However, Vaughan showed his experience and class to get over and Williams scrambled to touch the ball down when Saints erred on a kick as York went 12 ahead.
Welsby’s super off-the-cuff score was the try of the match as they went the length of the field but his side could not avoid another damaging defeat.
York, who had lost to bottom-of-the-table Huddersfield last week, face a trip to Leigh on Friday while St Helens will bid to try and inject life into their season against lowly Hull FC on Thursday.
York: Mataafa, Galeano, Williams, Bennison, Hunter, McShane, Martin, Inman, Va’a, Foster, Field, Hingano, Djeukessi.
Interchanges: Balmforth, Vaughan, Thompson, Buchanan.
Sin-bin: Hingano (46)
St Helens: Welsby, Murphy, Davies, Sailor, Douglas, Robertson, Hastings, Fifita, Burns, Walmsley, Wright, Host, Shorrocks.
Interchanges: Sinclair, Humphreys, Keeley, Matagi.
Referee: Liam Rush.