Green’s Dictionary of Slang

an excl. of approval, often in ironic use.

1899		W.J. Kountz Billy Baxter’s Letters 41: Wasn’t it immense where the main lady spurned the leering villain’s gold, and exclaimed with flashing eye, ‘Rags are royal raiment, when worn for virtue’s sake.’ Great!
1910		‘O. Henry’ ‘The Fifth Wheel’ in Strictly Business (1915) 69: ‘Great!’ said Thomas earnestly.
1915		H.L. Wilson Ruggles of Red Gap (1917) 352: ‘Great!’ she exclaimed in a hearty American manner.
1926		Van Vechten Nigger Heaven 120: Great! cried Dick. Now you’re talking.
1951		M. Spillane One Lonely Night 8: Yeah, great.
1984		A. Sayle Train to Hell 84: Oh, great, great.
1999		Guardian 1 Oct. 21: Great! How’s he doing? Great! Hey, what’re you doing on the doorstep? Come in! Great!