Avoid using C Strings on C++ code to trim leading whitespace
Suggestions on your code:


Your code currently returns the input string unmodified if it contains nothing but spaces and tabs.  I'd say it should return an empty string.
You always use input.at(x) instead of input[x].  The former checks at runtime for out of range errors and in which case throws an exception, while the latter does not.  Your code ensures that you are calling them with valid indexes and no additional check is needed.
Your code begins with an if statement.

if ((input.empty()) ||
    ((input.at(0) != ' ') &amp;&amp; (input.at(0) != '\t'))) {
    return input;
}


This special case looks redundant because your else branch already handles input that is empty or does not start with ' ' or '\t'.
Parentheses are welcome to clarify operator precedence, but you are using too many parentheses IMHO.  I would rewrite it like this:

if (input.empty() || (input[0] != ' ' &amp;&amp; input[0] != '\t'))


I removed the parentheses around input.empty() (which is a postfix-expression), and input[0] != ' ' and input[0] != '\t' (which are equality-expressions).  I don't think people will take it wrong.
Your use of size_t instead of int is great.  In fact, std::string::size_type is guaranteed to be size_t.  However, you should use std::size_t instead of the unqualified size_t.  (See When using C headers in C++, should we use functions from std or the global namespace?)
s.substr(i) is equivalent to s.substr(i, s.size() - i), so instead of

return input.substr(pos, (input.length() - pos));


It suffices to write

return input.substr(pos);

You store the value of i to pos, break the loop, and then construct the return value based on pos.  This looks unnecessary to me.  Why don't you just directly return inside the loop when the desired i is found?


With these applied, your code is already only three lines:

for (std::size_t i = 0; i &lt; input.length(); i++)
    if (input[i] != ' ' &amp;&amp; input[i] != '\t')
        return input.substr(i);


(You still need to fix the trim("    ") == "    " bug.)

In fact, you can simply make use of the C++ standard library facilities and simplify your code even further:

std::string trim_left(const std::string&amp; input)
{
    if (auto p = input.find_first_not_of(" \t"); p != std::string::npos)
        return input.substr(p);
    else
        return "";
}


If you cannot use C++17, move the declaration of p out of the if statement.