It is possible to jump start a law mower from a car. However, some lawn mower manufacturers recommend not jump starting mowers that have fuel injection, due to the possibility of damage to the electronics. If such is the case, then the battery must be removed and charged.
The same process of jump starting a car should be used in jump starting a lawn mower that does not have fuel injection. Using a set of jumper cables, connect the positive clamps to the positive terminals, which are usually marked "pos" or with a plus sign, and the negative clamps to the negative terminals, which are usually marked "neg" or with a minus sign.