Darrow's - Great pies, soups, whitefish, and the signature raspberry chicken salad. No atmosphere. At peak times there may be a fairly long wait.
Dixie - steaks, perch and the closest thing to a view at any restaurant in town.
Key Hole Bar - Good burgers, if you do not mind cigarette smoke. Otherwise the smoke may drive you out, but it may not be as bad at midday.
Chippewa Room at Audie's - Different menu than the main restaurant. Excellent, slightly upscale food at reasonable prices.
Depot Restaurant - burgers and appetizers in an historic railroad depot. One of the few restaurants in town with outdoor seating available.
Scalawags Whitefish & Chips - just a few tables or take out. Good whitefish, perch, & hush puppies . Just average French fries. This and Darrow's are the two that I would make a point of visiting.
Avoid Nonna Lisa's. My most recent experiences at 'Neath the Birches have been poor. Cunningham's is fairly good, but Darrow's does a better job in the same niche and isn't as crowded. Pancake Chef has a decent (if uninspired) evening buffet.
Stop at Big Stone Fishery on US-23 a mile east of town and get smoked fish and the smoked whitefish spread.
There is certainly no shortage of restaurants in Mackinaw, so this may seem harsh but... I would honestly say not to bother with anywhere but Darrow's. I know others have enjoyed Audie's and 'Neath the Birches -and these may be worth a try- but my experiences at both have been... meh. Particularly avoid The Dixie Saloon and Nonna Lisa's, which have appealing decor but appalling food.
That being said, you should know what to expect at Darrow's. It is strictly a family restaurant, nothing fancy. They are well-known for home cooking (roast beef, roast turkey, meatloaf, etc.) and PIE. They have at least 15 varieties of pie available every day. There is no alcohol. It may not be the most impressive meal of your life, but I'm pretty sure it will be the best meal you could have ,dollar for dollar, in Mackinaw. There is usually a long line at the door, but they do not take names - you just wait your turn. Things move pretty swiftly, so the wait isn't bad.
If you're looking for wine with dinner, or a more upscale menu, the Chippewa Room at Audie's might be more to your liking.
I'm going to totally agree with Tokyo. I will say this about Mackinaw no matter where you go. It's a resort town. Things change every season and some times they change a bunch from early season to late season. In years past summer labor was all college kids. Now summer labor is from the Islands. People come and go very quickly and that impacts service and quality. Pay attention to how busy these places are. If it's 6 pm on the weekend in high season it's not a bad time to think about driving to the Douglas Lake Bar or just getting some KFC.
I'll leave a link to another thread that may be helpful. The only other thing I would add is that The Big Stone Fishery is a great place to pick up some fresh Whitefish or Perch. The Perch will be Canadian and you should know the staff here can be nothing short of darn surly but it's still a good stop for fresh fish.
For smoked whitefish, dip or jerky the best (IMO) is available at Kruegers fish market. Just a few doors South of Darrows. From down town head west on Main street past the IGA and under the I-75 over pass. Take the first right and it will be on the left hand side next to a gas station. You can't miss the smoke house. Only about a block from the Holiday Inn Express.
The Dixie Saloon is best avoided at all times.