To evaluate the series \((-1) + 2 + (-3) + 4 + \ldots + 120 + (-121)\), we can first reorganize the terms into pairs for easier calculation.

Observe the pattern:

- The series alternates between negative odd numbers and positive even numbers.
- The last terms of the sequence are \(120\) (which is positive) and \(-121\) (which is negative).

Let's pair the terms:

\[
(-1) + 2, \quad (-3) + 4, \quad (-5) + 6, \quad \ldots, \quad (-119) + 120
\]

The \(n\)-th pair of terms can be written as:

\[
(- (2n - 1)) + (2n) = 2n - (2n - 1) = 1
\]

Now, we need to determine how many such pairs we have. The terms range from \(-1\) (the first term) to \(-121\) (the last term), performing an alternating sum over the range of odd and even numbers.

The sequence of odd numbers from \(1\) to \(121\) (inclusive) contains \(61\) odd numbers:

\[
-1, -3, -5, \ldots, -121
\]

The sequence of even numbers from \(2\) to \(120\) (inclusive) contains \(60\) even numbers:

\[
2, 4, 6, \ldots, 120
\]

Thus, this gives us \(60\) complete pairs from \((-1 + 2)\) to \((-119 + 120)\), contributing \(60\) to the sum.

Since we also have the last term \(-121\) remaining after accounting for all pairs, we should add that to our previous result:

\[
60 + (-121) = 60 - 121 = -61
\]

Thus, the value of the series is:

\[
\boxed{-61}
\]