After a pulsating encounter at Rugby Park, Longstaff netted a minute into added time to see off Ross County by the odd goal in five.
Fellow Magpies loanees Freddie Woodman and Callum Roberts were also involved for Killie, who moved up to eighth in the standings with victory.
Longstaff wasn’t the only United player to bag a decisive goal on Saturday, as Adam Armstrong’s 70th-minute effort earned Barnsley a 1-0 Sky Bet Championship win over Rotherham United at the New York Stadium.
Despite selling two players to Aston Villa in the build-up to the clash, Armstrong’s strike helped the Tykes climb up to seventh – just one point shy of a play-off place.
In League One, Ivan Toney notched his first goal for Scunthorpe United as the Iron reclaimed their place at the top with a 3-2 win over Port Vale at Glanford Park.
Toney lobbed ex-Newcastle ‘keeper Jak Alnwick from the edge of the penalty area just after the hour-mark, Graham Alexander’s charges a point clear of Sheffield United with a game in hand.
Elsewhere in the third tier, Alex Gilliead came on as a late substitute as Bradford City won 2-1 at struggling Oldham Athletic.
A division below, Macaulay Gillesphey played all 90 minutes as Carlisle United’s stuttering form continued with a 1-1 draw at home to ten-man Barnet.
It was a far more satisfying afternoon for young goalkeeper Paul Woolston, however, who helped Blyth Spartans to a 4-1 win over Matlock Town in the Evo-Stik Northern Premier League.
With the score at 1-1, Woolston pulled off a superb close-range stop to deny Matlock’s Edward Cribley, before Spartans ran riot in the final half-hour.
On the continent, Florian Thuavin bagged his sixth goal of the season in Marseille’s 5-1 rout of Montpellier at the Stade Vélodrome.
Since arriving back in his homeland, Thauvin has racked up almost 25 appearances for L’OM, who currently sit sixth in Ligue 1.
In the Netherlands, Siem de Jong entered the fray late on as PSV Eindhoven recorded an important 2-1 win against Heracles.
Kevin Mbabu, meanwhile, played 65 minutes for BSC Young Boys, who warmed up for the return of the Swiss Super League with a 1-1 friendly draw with Neuchatel Xamax.
Neither Tim Krul, Emmanuel Riviere nor Henri Saivet figured for their respective clubs – the latter having still been on international duty with Senegal at the Africa Cup of Nations.
OUT ON LOANAdam Armstrong – Barnsley (end of season)Macaulay Gillesphey – Carlisle United (end of season)Alex Gilliead – Bradford City (end of season)Ivan Toney – Scunthorpe United (end of season)Paul Woolston – Blyth Spartans (work experience loan)Sean Longstaff – Kilmarnock (end of season)Callum Roberts – Kilmarnock (end of season)Freddie Woodman – Kilmarnock (end of season)Siem de Jong – PSV Eindhoven (season-long loan)Tim Krul – Ajax (season-long loan)Kevin Mbabu – BSC Young Boys (season-long loan)Emmanuel Riviere – Osasuna (season-long loan)Henri Saivet – Saint-Etienne (season-long loan)Florian Thauvin – Marseille (season-long loan)
