The 25-year-old has enjoyed three excellent years with the Arsenal Ladies.
She helped the Gunners to back to back Super League titles in 2011 and 2012, two FA Cups, three Continental Cup titles and three consecutive Champions League semi finals.
Arsenal Ladies General Manager Vic Akers said: "Steph's a great player and she's been fantastic for us since signing from Leeds back in the summer of 2010. We're sad to lose her of course, but we wish her all the best for her time at Manchester City."
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