How do I convert a list in a Pandas DF into a string?

You should certainly not convert to string before you transform the list. Try:

df['col'].apply(', '.join)

Also note that apply applies the function to the elements of the series, so using df['col'] in the lambda function is probably not what you want.

Or, there is a native .str.join method, but it is (surprisingly) a bit slower than apply.

When you cast col to str with astype, you get a string representation of a python list, brackets and all. You do not need to do that, just apply join directly:

import pandas as pd

df = pd.DataFrame({
    'A': [['a', 'b', 'c'], ['A', 'B', 'C']]
    })

# Out[8]: 
#            A
# 0  [a, b, c]
# 1  [A, B, C]

df['Joined'] = df.A.apply(', '.join)

#            A   Joined
# 0  [a, b, c]  a, b, c
# 1  [A, B, C]  A, B, C