Green’s Dictionary of Slang

johnny-on-the-spot adv.

(US) promptly.

1957		‘Ed Lacy’ Lead With Your Left (1958) 73: Soon she died they had a lawyer here johnny-on-the-spot claiming the estate.
2011		‘Jack Tunney’ Split Decision [ebook] Whit’s driver Johnny-on-the-spot to pick up the boss man.