My async call is returning before list is populated in forEach loop

This code

Future<List<String>> readHeaderData() async {
  List<String> l = new List();
  List<String> files = await readHeaders(); // Gets filenames
  files.forEach((filename) async {
    final file = await File(filename);
    String contents = await file.readAsString();
    User user = User.fromJson(json.decode(contents));
    String name = user.NameLast + ", " + user.NameFirst;
    print(name);
    l.add(name);
  }
  return l;
}

returns the list l and then processes the asyc forEach(...) callbacks

If you change it to

Future<List<String>> readHeaderData() async {
  List<String> l = new List();
  List<String> files = await readHeaders(); // Gets filenames
  for(var filename in files) {  /// <<<<==== changed line
    final file = await File(filename);
    String contents = await file.readAsString();
    User user = User.fromJson(json.decode(contents));
    String name = user.NameLast + ", " + user.NameFirst;
    print(name);
    l.add(name);
  }
  return l;
}

the function will not return before all filenames are processed.

files.forEach((filename) async {

means that you can use await inside the callback, but forEach doesn't care about what (filename) async {...} returns.

Also possible

await Future.forEach(yourList, (T elem) async { ...async staff });