The incident occurred in the 68th minute when Adam Tann and Truro forward Isaac Vassell tangled in the area, and the referee pointed to the spot when the visitors felt they should have been awarded a free kick.
Shane White duly stepped up to slot home the resulting penalty and that ultimately proved enough to secure his side promotion.
St Neots also had a goal chalked off in the 86th minute when Lee Clarke was deemed to have handled Lewis Hilliard's cross into the net from close range.
Truro made the brighter start to the match and Sam Harty was forced to clear off the line as early as the third minute.
At the other end, Clarke had a deflected shot saved and Drew Roberts was denied when goalkeeper Tom McHale dived at his feet to make the stop.
Then Brett Longden was in the right place at the right time to clear off the line from Truro forward Matt Wright after he had beaten Saints stopper Paul Bastock.
St Neots' under-18s striker Ebey Marango was brought on for Matt Spring at the interval and St Neots made some inroads after the break, with Roberts forcing a save from McHale and Clarke rattling the woodwork in the 63rd minute.
But five minutes later, Truro wrested the advantage when White scored from the penalty spot.
St Neots switched to playing three at the back and pushed forward in an effort to find an equaliser.
But Clarke's disallowed effort was as close as it got, and it was Truro who were celebrating promotion at the final whistle.
Truro City: McHale, White, Brett, Ash, Farkins, Palmer, Green, Krac, Wright (Hayles, 90min), Vassell (Duff, 73), Cooke (Afful, 63).
Subs not used: Moore, Pugh.
St Neots Town: Bastock, Tann, Longden, Mitchel-King, Frater, Ward, Harty, Spring (Marango, 46min (Ferrari, 85)), Clarke, Roberts, Hilliard.
Subs not used: Greygoose, Bickerstaff, Viera.
Referee: Mr A Penwell.
Read more: http://www.cambridge-news.co.uk/Truro-City-1-St-Neots-Town-0-Saints-8217/story-26439668-detail/story.html#ixzz3ZSDxZgZx
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