I lost 160lbs in 2 years with only diet and exercise . IMHO it was about 90% diet and 10% exercise. Very overweight people won't be physically able to exercise enough to see significant long-term improvements without strict dieting.
If you just need to shed a few pounds, your diet isn't likely too bad to start with and some additional exercise may do the trick fairly easily. On the other hand, if you are 100 lbs. overweight, your diet is likely the main factor why, and should be addressed as the main problem.
A person's diet and nutrition have to always be a top priority. Even if you can get lean by simply consuming fewer calories than you burn, if you don't get the proper nutrition over the long term your body can get sick or injured and all that exercise benefit will soon be lost.