The moment generating function of an exponential random variable X is given by mgf(t) = m/(m - t)

That is, mgf(t) = E(exp(t*X)) = m/(m - t)

Let X_1, X_2, .... X_n are iid random variables having exponential distribution with parameter m.

Let m = 2;

and t = 0.5;

n = 10000

I wish to get an estimate of mgf(0.5) using sum_{i = 1 to 10000} exp(0.5 * X_i) as given my code below:

size = 10000;

m = 2;

xVal = exprnd(m,[1,size]);

t = 0.5;

tVal = t * xVal;

eVal = exp(tVal);

avgVal = sum(eVal)/size

actVal = m/(m - t)

the actual value is 4/3 = 1.333 given by actVal

When I run my code the estimate avgVal churns out results such as
9.9936, 12.9107, 10.6934

when it should ideally give values close to 1.333 according to the law of large numbers.

I face similar problems for quite a few values of m and t.

Where am I going wrong??

Yes, but the value you must to generate the exponential should be 1/m instead of m. The population mean of an exponential distribution with parameter m is 1/m. In the matlab exprnd(mu,...) function, mu is supposed to be the mean. When you factor this, you get it right.
Hope it was clear.