 McMahon studied at the University of Wollongong and obtained a Bachelor of Commerce in 1997 and a master's degree in 2005.She was the senior industrial relations officer for BlueScope prior to her nomination as a candidate for the 2007 New South Wales election to replace the sitting Member for Illawarra Marianne Saliba after an electoral redistribution.She was supported in her preselection by Morris Iemma.