Lady Montague dies after her son's exile. It is thought that she died of grief, as Montague states:

Prince.
Come, Montague; for thou art early up,
To see thy son and heir more early down.

Montague.
Alas, my liege, my wife is dead to-night;
Grief of my son's exile hath stopp'd her breath:
What further woe conspires against mine age?

Romeo's mother, Lady Montague 5 iii 209. Montague tells the prince that she died that very night.

Lord Montague tells us that Lady Montague dies after she finds out that Romeo has been banished. Her character doesn't have a big part in the play, so students sometimes overlook her death in the play.