Crawler for performing reverse search and writing results into a CSV file
There are multiple things we can do to improve the code:


variable naming - try to be consistent with PEP8 naming suggestions - for instance: 


Page should probably be page - or even better url
Address would be address
Contact would be contact
f can be input_file
g can be output_file

titles variable is never used
move the url format string into a constant
you don't need writer = csv.writer = csv.DictWriter(...) - just assign the writer to the DictWriter instance directly
since you are crawling the same domain, re-using requests.Session() instance should have a positive impact on performance
use .findtext() method instead of xpath() and then getting the first item
I would also create a separate crawl function to keep the web-scraping logic separate


Here is the modified code with the above and other improvements combined:

import csv

import requests
from lxml import html


URL_TEMPLATE = "https://www.yellowpages.com/los-angeles-ca/mip/{}-{}"


def crawl(entries):
    with requests.Session() as session:
        for entry in entries:
            url = URL_TEMPLATE.format(entry["Name"].replace(" ", "-"), entry["Lid"])
            response = session.get(url)
            tree = html.fromstring(response.text)

            titles = tree.xpath('//article[contains(@class,"business-card")]')
            for title in titles:
                address = title.findtext('.//p[@class="address"]/span')
                contact = title.findtext('.//p[@class="phone"]')

                print(address, contact)

                entry['Address'] = address
                entry['Phone'] = contact
                yield entry


if __name__ == '__main__':
    with open("predefined.csv", "r") as input_file, open('newly_created.csv', 'w', newline='') as output_file:
        reader = csv.DictReader(input_file)
        field_names = reader.fieldnames + ['Address', 'Phone']

        writer = csv.DictWriter(output_file, fieldnames=field_names)
        writer.writeheader()

        for entry in crawl(reader):
            writer.writerow(entry)


(not tested)