Sorting an Array of JavaScript Objects a Specific Order (using existing function)

Just use indexOf to convert the key to the correct order:

var order = ["c", "a", "b", "d"];
_.sortBy(arr, function(obj){ 
    return _.indexOf(order, obj.key);
});

Fiddle

If there are a lot of keys, then it would be advantageous to make a hash-map out of the array, like:

var order = ["c", "a", "b", "d"];
var orderMap = {};
_.each(order, function(i) { orderMap[i] = _.indexOf(order, i); });

This makes the key-sorting lookup constant time rather than O(n). (Fiddle)