actedas if she was a magistrate, but its important to note that she was not one, one ;-)

The law in England and Wales did not allow women to become serving magistrates( Justices of the Peace) until 1920, with the implementation of the Sex Disqualification (Removal) Act 1919. This act came into force on 23 December 1919,and it was in ealry 1920 that women were first appointed as magistrates.

JA actually makes this quite clear in that paragraph about Lady Catherine:

Elizabeth soon perceived that, though this great lady was not in the commission of the peace for the county, she was a most active magistrate in her own parish, the minutest concerns of which were carried to her by Mr. Collins; and whenever any of the cottagers were disposed to be quarrelsome, discontented, or too poor, she sallied forth into the village to settle their differences, silence their complaints, and scold them into harmony and plenty.

The phrase being on "the commission of the peace for the county" is used to describe the men who held the office of magistrate, or as it was alternatively known Justice of the Peace.