The 14-year-old was stuck in the Keer channel near Warton stock car track.
Morecambe lifeboat crew, with help from a coastguard team, found the girl after a search and dug her out in a 40-minute operation.
She had been in the mud for 90 minutes and was suffering from cold and possibly serious complications associated with suspension trauma due to the length of time that she had been trapped.
The girl was transferred from the Morecambe hovercraft to the coastguard helicopter and flown to the Royal Lancaster Infirmary for treatment.
The call to the lifeboat crew came at 6.50pm on Wednesday and the girl was found at 7.30pm.
A second girl who was reported missing was found safe and well.
“This was a very serious incident but thankfully, with good inter-agency cooperation, we were able to achieve a good outcome,” said Kevin Jackson, Morecambe RNLI Lifeboat Operations Manager.