Hampered by injuries and lack of confidence, the Welsh winger has often had to take a back seat to younger rivals. At Castleford, he was preferred to the teenage protege, Danny Arnold, and responded with the two tries that preserved Saints' advantage at the top.
His first, after Paul Newlove had already given Saints the lead, was pure opportunism, after he spotted a gap and romped through from dummy half.
Castleford, showing their best form of the season in recent weeks, would not be shaken off, with Frano Botica's accurate kicking and Jason Flowers' try bringing them level by half-time.
Sullivan then showed all his powers of anticipation to get on the end of Bobbie Goulding's kick to put his side ahead once more early in the second half.
When their substitute Tommy Martyn also went over, Saints had a 10 point lead and looked safe.
Matters are rarely as simple with St Helen's, however, and Tony Smith's try put them under severe pressure for the last 10 minutes.
It was only by a slender margins that they held firm, Steve Prescott's desperate tackle stopping Flowers on his way to what would have been a winning try.
Like Sullivan, Prescott can reflect on playing a crucial part in Saints' tightrope walk towards the Championship.
Castleford: Flowers; C Smith, Goddard, Chapman, Flynn; Botica, T Smith; Crooks, Tuuta, Sykes, Schick, Harland, Smales. Substitutes: Gay, Paramore, Sampson, Richardson.
St Helens: Prescott; Hayes, Matautia, Newlove, Sullivan; Hammond, Goulding; Perelini, Cunningham, Fogerty, McVey, Pickavance, Morley. Substitutes: Haigh, Martyn, Busby.
Referee: D Campbell (Widnes).Reuse content