Brown made her county debut in 2006, for Lancashire against Somerset.[3] Brown's first standout year came with the ball in 2008, where she took 8 wickets at an average of 22.62 in the County Championship, including 4/31 taken against Somerset.[4][5] In 2010 she again did well with the ball, taking 12 wickets in the County Championship at an average of 18.58.[6] In 2011, Brown made her maiden county century, scoring 101 against Devon, and overall scoring 250 runs at an average of 31.25, the most for her side.[7][8] In 2012, she was her side's leading run-scorer and leading wicket-taker in the County Championship, with 198 runs and 12 wickets, including her List A best bowling of 5/9, taken against Wales.[9][10][11]
In the 2013 and 2014 County Championships, Brown again passed the 200-run mark, and in 2014 made her second county century, scoring 112* against Cheshire.[12][13][14] In 2015, she was again her side's leading run-scorer in the County Championship, with 156 runs, and in 2016 she was Lancashire's leading wicket-taker, with 9 wickets at an average of 12.77, in the Twenty20 Cup.[15][16]
In 2017, Lancashire won the double of the County Championship and the 2017 Women's Twenty20 Cup, with Brown appearing in every match.[17][18] Personal highpoints came mainly in the Twenty20 Cup, where she hit her T20 high score of 70 against Middlesex, and took 9 wickets at an average of 9.44.[19][20] The next season, Brown took 7 wickets at an average of 13.14 in the Twenty20 Cup.[21] She played four matches for the side in the 2021 Women's Twenty20 Cup, scoring 31 runs and taking 1 wicket.[22][23] In the 2022 Women's Twenty20 Cup, Brown achieved both her Twenty20 high score and best bowling figures, scoring 80 against Cumbria and taking 4/21 against Derbyshire.[24][25]