I'm not quite sure, as I've never thought about it. But when I think about it it seems consistent that they are added to chronological to /etc/mtab. So you believe you should be able to retrieve it like this:
tail -n 1 /etc/mtab
mount | tail -n 1
If you want the mount point, you could change them to something like this:
tail -n 1 /etc/mtab | cut -d ' ' -f 2
mount | tail -n 1 | cut -d ' ' -f 2
To filter out the mountpoint. If you want the device, change the cut field accordingly (the -f option). Does this seem to work for you?