Asynchronously assembling a complex object
As for me it looks fine except one thing. You have already completed Task in .ContinueWith block, thus you may use .Result then instead of await and avoid redundant async State Machine.
By the way, if you're using async/await, probably there's no continuations needed.
Consider this continuations-free implementation of 1st Task.WhenAll() job.
public async Task&lt;Account&gt; Retrieve(AccountId input1, DateTime input2)
{
    var account = new Account();

    // probably try-catch wrapper maybe useful here as replacement of !mapTask.IsCompletedSuccessfully
    await Task.WhenAll((Func&lt;Task&gt;)(async () =&gt;
    {
        T_AccountMapper map = await _tAccountMapperRepo.Retrieve(input1);
        // continuation:
        AccountMap accountMap = new AccountMap(map.AccountId, map.Login, map.Namespace);
        account.SetAccountMap(accountMap);
    })
    //, (Func&lt;Task&gt;)(async() =&gt; {...})
    );

    return account;
}

As you can see, TranslateAccountMap method isn't needed anymore.
Finally, it's up to you.
P.S. Here's a useful link.